1. Field of the Invention
This invention relates to a synchronous machine and, in particular, to a synchronous machine having an inner rotor excited by permanent magnets.
2. Description of the Prior Art
Heretofore, in synchronous machines excited by permanent magnets, a large stray flux had to be tolerated in the magnetic circuit of the rotor because of the limited back magnetizability of the ferrites used for the permanent magnets. Due to the presence of such a large stray flux the degree of utilization of the machine was significantly reduced. Recently, with the discovery of ferrite magnets with high coercive force, it has become possible to realize a synchronous machine having reduced laminations as well as a low stray flux. However, while such a machine exhibits a higher torque per active machine volume, it also requires the use of an amagnetic shaft in the form of a double-T section. Such a shaft is relatively expensive to manufacture, so that the use of the machine becomes economically justifiable only at high speeds (up to 20,000 RPM) and, therefore, at large centrifugal forces.
It is a primary object of the present invention to provide a synchronous machine having a rotor which permits high machine utilization as well as the use of a conventional ferromagnetic shaft of simple design for rotor speeds which are not higher than 10,000 RPM.